Conjugated diene polymers are used as raw material rubbers in a variety of fields. For example, conjugated diene polymers are widely used in tires because these polymers advantageously provide treads having high resistance to wear and high flexibility at low temperatures and carcasses having low heat-generating properties and high flex crack resistance.
Among these, it is known that natural rubber having a cis-1,4-bond content (hereinafter also referred to as “cis content”) of approximately 100% and conjugated diene polymers having high tacticity such as polybutadienes having a cis-1,4-bond content of 98% or more are crystallized during drawing to attain high tensile strength (for example, Non-Patent Literature 1). These highly tactic conjugated diene copolymers have been produced with a coordination polymerization catalyst. A variety of catalyst systems such as Ti, Co, and Ni has been known. Conjugated diene polymers having a high cis content have been also produced with these catalyst systems in an industrial scale (for example, Non-Patent Literature 2).
In the late 1970s, systems using rare earth catalysts such as rare earth carboxylates have been proposed. Typically, the rare earth carboxylate catalyst systems have a cis content of approximately 94 to 97%. These catalyst systems have higher polymerization activity than those of the traditional Co, Ni, and Ti catalysts at a polymerization temperature of 60 to 80° C. or more. For this reason, these catalyst systems enable polymerization at higher temperatures and have narrower molecular weight distribution, which are characteristics not found in the traditional catalyst systems (for example, Patent Literatures 1 and 2 and Non-Patent Literature 3).
As an attempt to increase the cis content, a system using a binuclear complex of a rare earth element and alkylaluminum has been proposed, which has shown high cis selectivity at a cis content of 99% or more in the polymerization of isoprene (Non-Patent Literature 4).
Another rare earth binuclear complex has been proposed, which is prepared by a reaction of a binuclear complex of a rare earth element coordinated with toluene and alkylaluminum halide with alkylaluminum, and attains polymers having a cis content of 88 to 98.8% (for example, Patent Literature 3).
Furthermore, a system using a metallocene complex of a rare earth element has been proposed. This system attains polymers having a cis content of 96 to 100% (for example, Patent Literatures 4 to 8).